Interleukin-2 (IL-2) stimulates T lymphocytes to proliferate and is required for cell-mediated immune responses. The mechanism by which IL-2 triggers proliferation is poorly understood. As is the case with many other growth factors, an early intracellular response to IL-2 is the activation of a protein tyrosine kinase (PTK), which in turn may regulate the activity of a variety of cytoplasmic signal transducers. The receptor for IL-2 (IL-2R) does not have intrinsic PTK activity and likely couples to a nonreceptor PTK. The goal of the current proposal is to delineate the intermolecular associations that allow the IL-2R to activate cytoplasmic signal transducers. Among the signal transducers that are activated by the IL- 2R is phosphatidylinositol 3'-kinase (PI3-K). The proposed studies will focus on the coupling of the IL-2R to PI3-K and will address the following hypothesis: During the cellular response to IL-2, tyrosine phosphorylation of the IL-R beta chain induces a direct physical association between IL-2R beta and the p85 component of PI3-K, thereby facilitating the activation of PI3-K. I have demonstrated that a tyrosine-phosphorylated protein of relative molecular weight (Mr) 80 - 90 kD (pp80) binds to a fusion protein containing the SH2 domains of PI3-K p85, in lysates from IL2-stimulated (but not quiescent) T lymphoblasts. The specific aims of this proposal are: (1) To determine whether pp80 is the IL-2R beta chain and whether endogenous PI3-K p85 coprecipitates with IL-2R beta. (2) To characterize the interaction between pp80 and PI3-K p85. (3) To determine whether IL-2 stimulation leads to an association between PI3-K p85 and a protein kinase and whether p85 itself is phosphorylated in response to IL-2. (4) To determine if IL-2R stimulation leads to associations between PI3-K p85 and molecules that are not tyrosine-phosphorylated.